From a5804fcf7b22fc7d1f9ec794dd284c7d504bd16b Mon Sep 17 00:00:00 2001 From: Omar Sandoval Date: Thu, 14 May 2020 08:06:43 +0200 Subject: 9pfs: local: ignore O_NOATIME if we don't have permissions QEMU's local 9pfs server passes through O_NOATIME from the client. If the QEMU process doesn't have permissions to use O_NOATIME (namely, it does not own the file nor have the CAP_FOWNER capability), the open will fail. This causes issues when from the client's point of view, it believes it has permissions to use O_NOATIME (e.g., a process running as root in the virtual machine). Additionally, overlayfs on Linux opens files on the lower layer using O_NOATIME, so in this case a 9pfs mount can't be used as a lower layer for overlayfs (cf. https://github.com/osandov/drgn/blob/dabfe1971951701da13863dbe6d8a1d172ad9650/vmtest/onoatimehack.c and https://github.com/NixOS/nixpkgs/issues/54509). Luckily, O_NOATIME is effectively a hint, and is often ignored by, e.g., network filesystems. open(2) notes that O_NOATIME "may not be effective on all filesystems. One example is NFS, where the server maintains the access time." This means that we can honor it when possible but fall back to ignoring it. Acked-by: Christian Schoenebeck Signed-off-by: Omar Sandoval Message-Id: Signed-off-by: Greg Kurz --- hw/9pfs/9p-util.h | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'hw/9pfs') diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h index 79ed6b2..546f46d 100644 --- a/hw/9pfs/9p-util.h +++ b/hw/9pfs/9p-util.h @@ -37,9 +37,22 @@ static inline int openat_file(int dirfd, const char *name, int flags, { int fd, serrno, ret; +again: fd = openat(dirfd, name, flags | O_NOFOLLOW | O_NOCTTY | O_NONBLOCK, mode); if (fd == -1) { + if (errno == EPERM && (flags & O_NOATIME)) { + /* + * The client passed O_NOATIME but we lack permissions to honor it. + * Rather than failing the open, fall back without O_NOATIME. This + * doesn't break the semantics on the client side, as the Linux + * open(2) man page notes that O_NOATIME "may not be effective on + * all filesystems". In particular, NFS and other network + * filesystems ignore it entirely. + */ + flags &= ~O_NOATIME; + goto again; + } return -1; } -- cgit v1.1